Recessed light fixtures, often called "pot lights", are in widespread use in residential and commercial premises. They appeal to designers both for the fact that they take up virtually no space in a room, being recessed into the ceiling, and for their aesthetic versatility and appeal.
A typical recessed light fixture comprises a housing, which is usually cylindrical, a socket for receiving a floodlight, halogen bulb or other lamp, and a housing trim for aesthetic purposes which is affixed over the mouth of the housing after the housing has been mounted into the ceiling and covers the housing flange.
Conventionally the housing is attached either directly to a joist or to cross-piece nailed between joists specifically for supporting the light fixture, or is provided with a mounting frame that is affixed to adjacent joists. Affixing the housing directly to a joist can be difficult, because the confined space within the housing does not allow for a screw or nail driving tool to approach at the proper angle, and alignment of the fixture so that it is flush with the ceiling can therefore be difficult to achieve.
If the fixture is to be mounted in the ceiling other than immediately beside a joist, either a cross-piece or a mounting frame is required to support the fixture. This presents the disadvantage during construction of a room that additional materials and labour are required to mount the fixture, and after the fixture is mounted the finishing panels (eg. wall board, ceiling tile etc.) must be measured so that an opening for the fixture is cut in the correct position, before the panel is affixed to the ceiling. Since the opening in the finishing panel must be barely larger than the diameter of the housing, in order to ensure that the housing flange will properly abut the ceiling, there is little margin for error.
Even more difficulty is encountered in retrofitting a recessed light fixture into a finished ceiling. A large piece of the ceiling panel must be cut away so that the cross-piece or mounting frame can be secured to the joists. The ceiling panel must be replaced once the housing is properly mounted, with a piece having a hole which is just larger than the housing diameter and cut in the proper position. This is a very labour-intensive process.
It would be advantageous in all of these situations to be able to mount the fixture without having to secure it to a joist, cross-piece or mounting frame. However, the cylindrical shape of the housing makes this problematic, because one must pass it through the opening cut for the fixture before securing the housing in place, and if the opening has been cut properly there is no clearance left around the housing to work within the ceiling.